Cationic hypophosphite compounds, such as nickel and ammonium hypophosphite, are rather expensive materials which have been used in electroless nickel plating baths along with hypophosphorous acid in order to eliminate both sodium and sulfate ions from these baths and extend the life thereof. In order to justify the increased cost of processes for producing nickel or ammonium hypophosphite so as to warrant this commercial utilization various methods have been devised, including most recently that of U.S. Pat. No. 5,522,972 assigned to the assignee of the present application.
In addition, efforts have also continued to produce nickel hypophosphite utilizing a crystallization process. This generally includes reacting hypophosphorous acid with nickel hydroxide or nickel carbonate, or some other suitable nickel compound. The problem with this method is the high cost and limited availability of hypophosphorous acid, as well as the expense of the nickel compounds as compared to the cost of nickel sulfate in commercial use.
Czech Patents Nos. 139,245 and 152,901 describe methods for producing nickel hypophosphite which do not use the expensive hypophosphorous acid. In these patents, nickel nitrate in an aqueous solution, is mixed with aqueous sodium hypophosphite. The nickel hypophosphite crystallizes out of solution because it is the least soluble material in the mixture and is easily separated. The filtrate is presumably discarded as waste. This is a very simple method for producing nickel hypophosphite. However, there is sufficient sodium hypophosphite and nickel hypophosphite left in the filtrate to cause this method to be very expensive. Thus, these expensive materials would be discarded in the waste stream, requiring expensive clean-up treatment. Similar processes are disclosed in Japanese Patent Nos. 97/02,809 and 97/02,810, which patents thus disclose contacting nickel hydroxide or nickel carbonate with hypophosphorous acid in water at 40.degree. to 80.degree. C., followed by cooling to crystallize the nickel hypophosphite product, as well as reacting nickel chloride with sodium hypophosphite in aqueous solution, followed by crystallization to produce the nickel hypophosphite. In each of these processes the waste streams would contain nickel and hypophosphite ions as well as the corresponding sodium salt, such as sodium nitrate, when nickel nitrate is used as described in the Czech patents discussed above, or sodium chloride when nickel chloride is used, as described in the above-referred-to Japanese patent. Furthermore, whereas the Czech patent mentions that nickel sulfate can be used, this too would result in sodium sulfate being present in the waste stream, together with nickel and hypophosphite ions.
The search has therefore continued for a simpler and more economical and more efficient way to produce such cationic hypophosphites.